Jahn-Teller distortion, ferromagnetic coupling, and electron delocalization in a high-spin Fe–Fe bonded dimer

Abstract

Fe 2 (N,N’-diphenylformamidinate) 4 , 1 , first synthesized in 1994, is one of very few non-organometallic compounds with Fe–Fe distances, (2.46 Å) suggestive of an Fe–Fe bond. The electronic structure of 1 has been unclear because of its distorted D 2 geometry, as well as its reported S = 4 ground state. Computational investigations using DFT methods have shown that the D 2 geometry is the result of a Jahn-Teller distortion away from D 4 symmetry, in which the ground state would be orbitally degenerate. Broken symmetry methods have shown that ferromagnetic coupling between the two high-spin Fe(II) ions in 1 is a consequence of spin delocalization caused by a three-electron σ bond and a weaker three-electron δ bond between the Fe atoms. The relationship between ferromagnetic coupling and an Fe–Fe bond is established from results using hybrid functionals having variable amounts of Hartree-Fock exchange, which is found, surprisingly, to mitigate Fe–Fe bonding.